Phase change materials (PCMs), such as GeSbTe (GST), are able to be switched between amorphous and crystalline states upon the application of an electrical pulse or a laser pulse. Their material properties, such as conductivity, bandgap, and refractive index, are distinct in the two states. Due to this property, PCMs have been widely used to construct electronic nonvolatile memories.
PCMs may also be used for constructing optical devices, which have a wide range of applications, such as sensing, imaging, and cognitive optical networks. For example, optical switching, i.e., dynamic routing of light into different paths, is widely used in photonic integrated circuits. Current on-chip optical switches are mostly based on electro-optical or thereto-optical effects, which usually produce small refractive index perturbations (e.g., typically well below 0.01). Therefore, the resulting devices often have a large footprint and significant energy consumption. In addition, switching mechanisms based on electro-optic or thereto-optical effects are volatile, so a continuous power supply is often used to maintain the optical switching state, thereby further increasing the energy consumption.
In recent years, optical devices based on PCMs have emerged for on-chip switching and routing. PCMs can generate a large difference in the refractive index during phase transition. In addition, a phase transition in a PCM can be nonvolatile, thereby allowing self-holding or latching in the resulting optical switches in the absence of power.
Despite these attractive features, the performance of existing PCM-based photonic switches is typically compromised by the high optical absorption in traditional PCMs. The two most commonly used PCMs include VO2 and Ge2Sb2Te5 (i.e., GST 225), both of which suffer from excessive optical losses even in their dielectric states. For example, the extinction coefficient (i.e., imaginary part of the refractive index) of amorphous GST is about 0.12 at 1550 nm wavelength, corresponding to about 42,000 dB/cm attenuation, which is unacceptably high for many guided-wave device applications.